This invention relates to a method and apparatus for handling and cutting boards of material.
One particular use for the invention described herein relates to the production of printed wiring boards, and the invention will accordingly be explained in terms of that use with the recognition that those skilled in the art will appreciate the broader scope of application.
Printed wiring boards comprise a relatively flat, layer of electrically insulative base material bearing a pattern of electrically conductive strips or paths. Electrical components are mounted on the board and are interconnected with each other by the conductive paths. The term "printed wiring board", as used herein, is meant to include boards whether or not populated by electric components.
Printed wiring boards are manufactured by initially forming the component-receiving holes in, and etching the pattern of electrically conductive paths on, the base material to form a base laminate. The base material is secured during the manufacturing processes by tooling adapted to interface with the process equipment while accomodating the particular shape and size of the board. In order to accommodate the endless variety of board shapes and sizes while maximizing the use of standard sized tooling, the circuits for a plurality of printed wiring boards are etched onto a masterboard of base material which is subsequently cut into the smaller individual boards that will hold the components.
The masterboard has heretofore been cut into individual boards by means such as routing or shearing, and has typically been cut prior to component insertion for a variety of reasons. For example, routing typically requires a 1/8" or 1/4" path, making it difficult to fit the router between adjacent components of neighboring populated boards.
Additionally, routing is generally economical only if the boards can be stacked, permitting several masterboards to be cut simultaneously. The stacking of the printed wiring boards has accordingly required that the boards be unpopulated.
Shearing not only requires substantial clearance between adjacent electrical components of neighboring populated boards, but also subjects boards to mechanical shock which can dislodge or damage electrical components mounted on the masterboard.
Cutting has typically been performed by one of two methods. The first cutting method, sometimes referred to as the full cut, simply entails the cutting and separating of the individual boards from the masterboard after the electrically conductive paths have been etched. Following the cutting of the individual boards, the electrical components are inserted and soldered in place. The populated boards are then typically tested and passed on to be used in the electrical units for which they were made.
The aforementioned "full cut" method is cumbersome when populated printed wiring boards of varying sizes and shapes are to be produced automatically. The tooling fixtures used after the cutting process, during such operations as automatic component insertion, wave soldering, and the like, must be capable of interfacing with the particular printed wiring board and each machine performing a step in the post-cutting process. Accordingly, tooling fixtures must be available for each of the individual board sizes and shapes, and the tooling must be changed in mid-production to accommodate the individual boards rather than the relatively standard sized masterboard.
The change in tooling subsequent to the cutting operation imposes an unwanted expense on the manufacturer. Besides the cost of the tooling itself, there is also additional labor cost when tooling must be changed in mid-production. To decrease the cost, associated with the parts and labor required by the use of multiple tooling, it is desirable to produce individual printed wiring boards of any shape or size on an automatic production line utilizing tooling of a standard size and shape.
To avoid the tooling changes required subsequent to the "full cut" operation, a second cutting method, referred to as the tab cut method, provides a means by which the individual boards can be maintained as one integral masterboard during a number of post-cutting operations. In the tab cut method, the tooling is sized to fit a masterboard of standard size and shape. The cut about the perimeter of each individual board is incompletely made so that each of the individual boards remain connected to the masterboard by means of a plurality of narrow tabs, approximately 2-3mm wide. The individual boards can thereafter undergo automatic insertion of electrical components, wave soldering, etc., as a single unit, without the need to change tooling. The tabs may then be broken and the boards separated.
While reducing tooling change, the tab cutting method introduces its own set of limitations. For example, the remnants of the tab may need to be sheared off and the previously tabbed region reworked to meet established standards regarding edge quality of the board. Consequently, additional processing steps may be introduced, with related additional expense and the potential for electrical component damage.
In addition to creating added expense, shearing is becoming increasingly impractical. While shearing involves the cutting of a straight edge, the electronics industry is increasingly producing odd-shaped boards to accommodate more complex circuitry which must fit into equipment housings having limited space. Thus, it is becoming increasingly desirable to provide printed wiring boards with curved or complex shaped edges, rather than straight edges. Further, shearing subjects the board to potential component-damaging mechanical shock, and is therefore undesirable, particularly where more shock-sensitive digital and/or surface-mounted components are mounted on the board.